1. Field of the Invention
The present invention relates to an organometallic complex and an organic electroluminescent device using the same, and more particularly, to an organometallic complex that can emit light ranging from a blue region to a red region through triplet metal-to-ligand charge-transfer (MLCT) and an organic electroluminescent device using the organometallic complex as an organic layer forming material.
2. Description of the Related Art
Organic electroluminescent (EL) devices, which are active-emissive type display devices, emit light by a recombination of electrons and holes at a fluorescent or phosphorescent organic layer receiving a current. Organic EL devices are lightweight, have wide viewing angles, produce high-quality images, and can be manufactured by simplified processes. In addition, by using organic EL devices, moving images with high color purity can be formed with low consumption power and low voltage. Accordingly, organic EL devices are suitable for portable electric applications.
In general, an organic EL device includes an anode, a hole transport layer, an emission layer, an electron transport layer, and a cathode stacked sequentially on a substrate. The hole transport layer, the emission layer, and the electron transport layer are organic layers. The organic EL device may operate thorough the following mechanism. First, a voltage is provided between the anode and the cathode. Holes injected from the anode move to the emission layer through the hole transport layer, and electrons injected from the cathode move to the emission layer through the electron transport layer. In the emission layer, the holes and electrons are recombined, thus producing excitons. The excitons decay radiatively, thus emitting light corresponding to the band gap of a material.
Materials for forming an emission layer are divided into fluorescent materials using singlet-state excitons and phosphorescent materials using triplet-state excitons, according to an emission mechanism. The fluorescent material or phosphorescent material may form the emission layer, or the fluorescent material or phosphorescent material-doped host material may form the emission layer. When electrons are excited, singlet excitons and triplet excitons are generated in a statistics ratio of 1:3 (see Baldo, et al., Phys. Rev. B, 1999, 60, 14422).
When the emission layer is composed of a fluorescent material, triplet excitons that are generated in the host cannot be used. On the other hand, when the emission layer is composed of a phosphorescent material, both singlet excitons and triplet excitons can be used, and thus, the inner quantum efficiency of 100% can be obtained (see Baldo et al., Nature, Vol. 395, 151-154, 1998). Accordingly, the use of the phosphorescent material results in better luminance efficiency than the fluorescent material.
When an organic molecule contains a heavy metal, such as Ir, Pt, Rh, and Pd, spin-orbital coupling occurs due to a heavy atom effect, and thus, singlet states and triplet states are mixed. Thus, a forbidden transition occurs and phosphorescent light is effectively emitted even at room temperature.
Recently, highly effective green and red emission materials that use phosphorescence having the inner quantum efficiency of 100% have been developed.
For example, transition metal compounds that include a transition metal, such as Ir or Pt, have been developed. However, materials that are suitable for highly effective full-color display and low-consumption power fluorescent application are green and red emission materials only. In other words, blue phosphorescent emission materials have not been developed. Accordingly, a phosphorescent full-color device cannot be manufactured.
In order to resolve this problem, blue emissive materials (disclosed in WO 02/15645 A1 and US 2002/0064681 A1); organometallic complexes that contains a bulky functional group that can deform the molecular geometry to widen the gap between a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO) or a functional group with a high ligand field, such as a cyano group (disclosed in Mat. Res. Soc. Symp. Proc. 708, 119, 2002; and 3rd Chitose International Forum on Photonics Science and Technology, Chitose, Japan, 6-8 Oct. 2002.); an Ir complex, such as Ir(ppy)2P(ph)3Y where Y=Cl or CN (disclosed in US 2002/0182441 A1); and an Ir (III) complex containing a cyclometalating ligand and chelating diphosphine, Cl, and a cyano group (disclosed in US 2002/0048689 A1) have been developed.